Even if for whatever reason he doesn't have a huge part in the actual game (the campaign or side missions) I strongly believe he should be the primary or secondary antagonist in the Red Hood dlc, not only to pay tribute to Under the Red Hood but also because Black Mask and Red Hood are perfect foes. One wears a black mask and runs gotham's crime. The other wears a red mask and stops it. There's also the fact that black and mask both end in k, and red and hood both end in d. Their chemistry's perfect!